\(L(T)\)  represents the energy absorbed along distance L, there is only 37% (\(e^{-1}\) ) of initial absorbed energy left since the energy dissipates exponentially along the penetration, which is related to the structural properties of CO ice. At 12.5 K deposition temperature, CO ice presents an amorphous structure. As deposition temperature increases, the structure becomes partially crystalline with a phase transition at 20 K [1]}, [2]}. Below phase transition temperature, the structure of CO ice is the same, leading to a constant \(L(T)\) . The CO molecules are arranged in a nearly crystalline structure at higher deposition temperature, i.e. above 20 K, \(L(T)\)  is thus larger, as shown in Fig. REF (a). In addition, [3]} shows that the crystalline \(\alpha \) -CO is formed above 21 K, supported by the TEM images and the electron diffraction pattern, and the structure of CO ice does not only depend on the temperature because also the substrate used for CO ice deposition plays a role: the size of the three-dimensional CO island can reach \(\approx \)  200 nm when deposited on amorphous H\(_2\) O ice, while it is only \(\approx \)  50 nm on CO\(_2\)  ice. The substrate is CaF\(_2\)  for all deposition temperature configurations in this study. The substrate effect on the crystallization of CO ice is out of the scope of this paper The lower energy transfer length below 20K is related to the non-alignment of molecular dipole, hindering resonant transfer mechanisms based on dipole-dipole interactions. In addition, this parameter determines the photodesorption trend as a function of N\(_{re}\) (CO), which means that the photodesorption yield drops either dramatically or gradually at low and high deposition temperatures, respectively.
